This invention relates to application of a thermal barrier coating to the metallic components used in the construction of gas turbine engines, specifically to application of a ceramic-based thermal barrier coating to an oxidation/sulfidation resistant diffusion coating over a nickel or cobalt-based superalloy substrate.
It is well known that the thermodynamic efficiency of gas turbine engines increases with the temperature at which the turbine is operated and, therefore, that it is desirable to operate a gas turbine engine at the highest practical temperature. It is also well known that most gas turbine engines must operate in the ambient environment where the high temperature engine components are exposed to the oxidizing and corrosive effects of the ingested constituents of the ambient air and fuel. Accordingly, materials used to fabricate gas turbine components ideally should have both good high temperature mechanical properties and should exhibit a high degree of resistance to surface degradation such as by oxidation or sulfidation (hot corrosion) at high temperature. Nickel and cobalt-based superalloys possess exceptional high temperature mechanical properties, however, they are prone to degradation from the oxidizing and corrosive effects of the environment. As is well-known in the art, nickel and cobalt-based superalloys are superalloys in which nickel and cobalt, respectively, are the single greatest constituent by weight.
Oxidation and sulfidation (hot corrosion) resistant coatings have been applied to turbine hot-section components for many years with positive results. A popular oxidation/sulfidation resistant coating is a metallic diffusion coating, typically an aluminide, however, metallic diffusion coatings may also contain a variety of other protective elements including chromium, nickel, cobalt, silicon, and platinum-group metals (i.e. platinum, palladium, and rhodium) plus lesser amounts of strong oxide forming elements such as tantalum, hafnium and yttrium. The term "metallic diffusion coating" is well-known in the art and is described in the patent literature including in U.S. Pat. No. 3,716,398 to Stueber et al., and U.S. Pat. No. 3,617,360 to Levine. "Metallic diffusion coating" defines a class of coatings that are applied by exposing the article to be coated to a halide vapor carrying the coating material (typically an aluminide) in an inert or reducing atmosphere. As used herein, the term "metallic diffusion coating" specifically excludes MCrAlY coatings, which are members of the class of coatings known in the art alternatively as "overlay coatings" or "bond coatings."
As demands for ever increasing fuel efficiency mount, however, engineers are designing turbine engines to operate at temperatures that approach or even exceed the melting point of even the highest temperature superalloys. Accordingly, in addition to the oxidation/sulfidation resistant coating (hereinafter "diffusion coating"), many engine designs now require a second thermal barrier coating ("TBC") to reduce the surface temperature of the superalloy substrate. Typical TBCs include ceramics such as yttria-stabilized zirconia.
Methods for bonding the ceramic TBC to the superalloy substrate have been the subject of numerous patents. Because the ceramic TBC is relatively stable and non-reactive, chemical adherence of the TBC to most materials is poor. Additionally, the difference in thermal coefficients of expansion between the substrate and the TBC generate stress that tends to cause spallation of the TBC from the surface. Accordingly, most investigation and patents relating to bonding TBCs to substrates has focused on improving the weak chemical adherence of the TBC through the use of bond coatings and the like.
U.S. Pat. No. 4,32)1311 to Strangman discloses a bond coating comprising a layer of MCrAlY (where M is an element chosen from the group consisting of Fe, Ni, or Co or alloys thereof, Cr is chromium, Al is aluminum and Y is yttrium) deposited onto the superalloy substrate. The bond coating is deposited preferably by Electron Beam Physical Vapor Deposition (EBPVD). After the MCrAlY bond coating is aluminized, a ceramic TBC is applied also preferably by EBPVD to produce a columnar grain structure. The ceramic TBC is stated to have some degree of solid solubility in the aluminized MCrAlY, which is the basis for the chemical adherence of the TBC to the MCrAlY.
U.S. Pat. No. 4,399,199 to McGill, et al. discloses a substantially pure platinum bond coat applied between the substrate and the ceramic TBC.
U.S. Pat. No. 5,238,752 to Duderstadt, et al. discloses a nickel aluminide bond coating onto which the ceramic TBC is deposited, preferably by EBPVD.
U.S. Pat. No. 5,262,245 to Ulion, et al. discloses a new superalloy having the capability of forming a thermally grown alumina scale on its outer surface onto which a ceramic TBC is deposited directly, preferably by EBPVD.
Receiving substantially less attention has been the development of methods to improve the mechanical bonding of the TBC to the substrate. U.S. Pat. No. 5,236,745 to Gupta, et al. discloses an air plasma sprayed bond coat that is applied to the substrate to produce a surface roughness of preferably 200-600 microinches RA. The patent teaches that the bond coat is thereafter aluminized to improve the chemical bonding of the TBC to the bond coat while maintaining the surface roughness.
None of the prior art, however, discloses or suggests mechanical treatment of the substrate itself to eliminate the bond coating, while maintaining a suitable bond between the ceramic TBC and the diffusion coated substrate.
Accordingly, it is a principal object of the present invention to provide a ceramic TBC that is mechanically bonded to a diffusion coated superalloy substrate without the need for a bond coating.